The Confederate prisoners were lined up 15 paces from the Union firing squad. The order was given, and the six rebels died instantly. Five of them were shot through the heart, the Union officer in charge reported, adding that the execution was conducted to "my entire satisfaction."

So what if they were innocent POWs. A band of rebels had massacred captured Union soldiers and their commanding officer a few weeks before. Now, Union commanders just needed to select a Confederate officer for death, to complete the eye-for-an-eye transaction.

There was no gallantry to this bloody affair in 1864, no stirring charge worthy of Currier and Ives. It was but a dark footnote to the epic of the American Civil War. And it was just what the National Archives sought for the major exhibit that will debut Friday: "Discovering the Civil War."

The exhibit, designed to launch Washington's celebration of the coming 150th anniversary of the war years, seeks to explore more of the little-known aspects of the battle and glimpse some of the dimmer corners of the conflict that remade the country and that so many Americans think they know so well.

Yet 150 years later, the anniversary of the war that tore the nation apart finds a country that remains racially divided, politically fractured and historically split -- even over the causes and legacy of America's most wrenching conflict.

The governors of two Southern states, Virginia and Mississippi, sparked controversy this month by neglecting or sounding dismissive of the role of slavery in the war. And one noted Civil War historian says the nation might be too divided to properly mark the key unifying event in its history.

"I think it's going to be impossible to get all the American people to gather to commemorate a portion of American history that's so important to the country," said Virginia Tech's James I. Robertson Jr., who 50 years ago directed the U.S. Civil War Centennial Commission. "People just aren't that together anymore.

"The nation is far more polarized and politicized now than it was" in the centennial, he said. "Every subject seems to become an issue."

But another scholar disagreed. Princeton historian James M. McPherson said that the Civil War centennial coincided with the civil rights movement. "On the issue of race, I think there was much sharper polarization then than now," he said.

McPherson said recent uproars point to "the way in which the war still resonates in American culture."

"Issues having to do with race and slavery and regionalism and federalism -- all of those are hot-button issues in American politics and American culture, and the Civil War looms over all of them," he said.

The war, which began in 1861 and ended in 1865, claimed more than 600,000 lives -- 2 percent of the population then. Today, that would mean 6 million dead, historians say. One battle in 1862, near Sharpsburg, Md., killed four times the number of American casualties on D-Day in 1944.